Enamels have been used extensively for the decoration of glass. Many of these decorations include bright opaque colors, such as the decorations on soda bottles, or include the black band printed on the edges of automobile windshields. Some of the decorations are translucent, such as the imitation acid-etch coating seen on various liquor bottles.
A general drawback to the use of enamel layers made from glass frit paste is that it is presently difficult to obtain optically transparent enamel glass coatings. Instead, the enamels have a certain degree of haze that forms in the enamel coating during firing.
Ordinarily, glass frit (which will also be referred to herein as particles or grains) is produced by adding and mixing together various oxides or other glass raw material components, and heating the mixture to smelting temperatures to form a molten glass batch. The resulting glass batch is quenched rapidly through the glass transition temperature of the glass batch to produce an amorphous solid. Cooling is conducted by means such as roll quenching or water quenching. After cooling, the amorphous solid is milled to form glass frit of a particular size suitable for use in glass enamels. Milling is also referred to herein as “grinding” or “fritting.” The amorphous solid could be milled in water, in a solvent, or by a dry method such as air milling or jet milling. The wet frit is dried by heating to remove the water content. The resulting dry glass frit diffracts light to a high degree due to the many exhibited surfaces of the frit particles.
When used to form enamel coatings, the frit can be mixed with an organic carrier (e.g., an organic binder and solvent) to form an enamel paste. The paste is then applied on a substrate, dried to remove any solvent or liquid in the paste, and fired to burn off the organic binder and to fuse the frit particles/grains. The molten enamel is then cooled to form a substantially continuous enamel coating on the substrate.
Ordinarily, firing of an enamel paste on a substrate causes the glass frit particles in the paste to fuse together. As the glass frit particles are fused together by heating, the number of exhibited frit surfaces decreases. That is, the surfaces of adjacent frit particles do not remain distinct from one particle to the next, but rather the frit particles melt or fuse together to form a substantially continuous coating of softened enamel, which is then cooled to form a hardened glass enamel coating on the surface of the substrate.
In other words, as the frit particles are subsumed into each other during firing, many surfaces of the individual frit particles are no longer apparent because the frit particles at least partially fused into a single mass of softened glass enamel. This can be likened to melting ice cubes. The surfaces of the ice cubes disappear as the cubes melt to form a pool of water. If the ice cubes are not completely melted however, then the surfaces of the cubes or portions thereof, will still be present and will still refract light. Likewise, if all frit particles in an enamel paste are not completely fused together, then the surfaces of the particles that remain unfused after firing will still diffract light to a certain extent. The diffraction of light at the frit surfaces contributes to the amount of haze in the enamel coating.
The processing steps used to make glass frit and fired enamel coatings may impact the surface chemistry of the glass frit particles, which can inhibit the fusion of glass frit particles. The unfused surfaces of the frit particles will be referred to herein as “grain boundaries.” The grain boundaries produce haze in the resulting enamel coating by diffracting light. While not being bound to any particular theory, it is believed that these processing steps result in chemically bound water being introduced into the glass frit, carbonates and bicarbonates being deposited on the surface of the glass frit particles, and/or seed crystals forming in the glass frit. These contaminants lead to an increase in the amount of grain boundaries present in a fired enamel coating and a corresponding increase the amount of haze as described below.
It is believed that the processing steps used to make glass frit and enamel coatings may lead to atmospheric water being absorbed by the glass frit during storage, wherein the frit is often stored under atmospheric conditions before being mixed with an organic carrier to form an enamel paste. Upon firing, the chemically bound water in the glass frit tries to escape from the enamel bulk in the form of water vapor. This gas may not fully escape the enamel bulk during processing and when the enamel is cooled, the gas may become trapped in the enamel coating in the form of bubbles. The trapped bubbles diffract light and cause haze in the enamel coating.
The processing steps used to produce frit and enamel coatings may also lead to alkali, alkaline earth, and boron species leaching to the surface of the frit particles. More specifically, low melting glasses may have relatively large amounts of alkali metals (Li, Na, K, Rb, Cs), alkaline earth metals (Mg, Ca, Sr, Ba) and boron (B), which may leach to the surface of the frit particles as a result of the grinding process. Because some grinding processes use water as a medium for grinding, the water partially dissolves these alkali, alkaline earth, and boron species to thereby form the associated hydroxides of these species. After milling, the frit is dried to remove the water content and these dissolved hydroxide species precipitate on the surface of the glass frit particles. When these precipitated hydroxide species are exposed to regular atmospheric conditions during storage, the precipitated hydroxide species can react with carbon dioxide in the atmosphere to form carbonates and bicarbonates on the surface of the frit particles.
It is believed that carbonate and bicarbonate species inhibit fusion of the glass frit particles. That is, carbonates and bicarbonates are situated on the surface of the frit particles and stand between frit particles during firing, thereby disrupting the fusion of the particles. As such, some of the frit particles, or portions thereof, are prevented from completely fusing together. The grain boundaries that are present as a result of unfused frit will refract light and thereby contribute to haze in the enamel coating.
Furthermore, the presence of carbonate and bicarbonates on the surface of the glass frit particles could lead to the production of CO2 gas in the bulk of the molten enamel. CO2 gas could form from the decomposition of carbonates and bicarbonates during the firing process, causing the formation of carbon inclusions or gas bubbling in the molten glass enamel. These bubbles may be trapped in the cooled enamel coating upon cooling, which would further contribute to unwanted haze.
The processing steps used to produce frit and enamel coatings may also lead to the production of seed crystals in the glass frit. More specifically, as a result of the leaching of alkaline and alkaline earth species from the glass frit during grinding, higher amounts of the network former oxides and network intermediate oxides remain in the glass frit particles. Such higher amounts of these elements may yield seed crystal compositions of Zn2SiO4 or Bi4Si3O12, depending upon the glass composition. The seed crystals can act as nucleation sites for the growth of larger crystals during firing of the glass enamel. The seed crystals or larger nucleated crystals can inhibit fusion of the glass frit particles. Like carbonates and bicarbonates, the seed crystals stand between frit particles during firing and prevent the frit particles from completely fusing together. This results in light diffracting grain boundaries in the fired enamel coating that cause haze. Furthermore, the seed crystals and larger nucleated crystals can themselves diffract light and thereby increase haze in the fired enamel coating.
Because of the gas, carbonates and bicarbonates, and seed crystals present in the enamel pastes, some of the frit particles, or portions thereof, remain distinct and separate from adjacent frit particles even after firing. Therefore, the surfaces of the unfused frit particles, i.e. grain boundaries, diffract light and create haze in the fired enamel coating.
The amount of haze in the fired enamel coating is commensurate with the size or number of grain boundaries present in the fired enamel coating. In this way, it can be generally summarized that reduced fusing of frit particles results in more grain boundaries being present in the fired enamel coating, which results in more haze in the coating.